Radioactivity Dynamics in River System
（2025）
ＱWhy does the concentration of radiocaesium (137Cs) in coastal seafloor surface sediments vary by location and time?
ＡThe main causes are (i) the strength of near-bed water motion and (ii) differences in sediment grain size. Where waves and tidal currents are strong, sediments tend to be mixed and homogenized; in quiescent areas, episodes such as storms intermittently stir the seabed and sort grains, so 137Cs concentrations vary by location and time.
Monitoring after the Fukushima Daiichi Nuclear Power Plant accident has shown that 137Cs concentrations in seafloor surface sediments have decreased over time. However, within the coastal zone (water depth shallower than 30 m), there are sites where the decrease is slower and sites where concentrations are highly variable. Because the coastal zone is closely tied to fisheries and recreation, clarifying the drivers of variability contributes to reassurance in local communities.
A research team at the Japan Atomic Energy Agency investigated a shallow shoreface near the plant (a wave-agitated setting) and a somewhat deeper offshore/inner-shelf area (usually calm, mixed intermittently during storms) by collecting sediment cores during 2019–2021. They examined vertical (depth-dependent) 137Cs profiles and grain size (sand, silt, clay), and analyzed these together with near-seabed ocean conditions (waves, tides, and near-bed disturbance during storm events).
Findings
Shoreface (wave‑ and tide‑dominated): The upper seabed—down to several tens of centimeters—is well mixed. Within sandy layers, 137Cs is distributed relatively uniformly, and little temporal change is observed.
Offshore/inner shelf (usually calm, mixed during strong storms): Storms preferentially transport fine particles away and leave coarser grains behind, causing grain‑size sorting. Consequently, 137Cs concentrations can vary markedly depending on the sampling location and timing.
Figure: Schematic of near‑seafloor 137Cs dynamics in the coastal zone
Shoreface: tends toward uniformity and decreases slowly;
Offshore/inner shelf: event‑driven sorting of sand and mud leads to time‑ and depth‑dependent variability.
Related articles
Ｑ How is the concentration of radioactive cesium in seawater and marine sediment changing?
Ｑ How extent does radiocesium flowing in from rivers affect the coastal environment?
References